These studies are designed to study the influences of dietary protein on the whole body and fractional protein synthesis and the accretion of skeletal muscle mass during a high intensity progressive resistance training (PRT) program. Previous studies have demonstrated that older men and women retain the capacity to increase both muscle strength and size in response to PRT, even into the tenth decade of life. Our studies have also demonstrated that the metabolic response to exercise, particularly exercise that involves eccentric exercise, is quite different between old and young subjects. Compared to young, older subjects experience greater amounts of muscle damage, greater myofibillar protein breakdown, decreased neutrophilia and circulating creatine kinase activity. We have also demonstrated that regular exercise increases the need for dietary protein and a dietary supplement will favor the accretion of muscle mass in older men. This study leads us to the hypothesis that increased dietary protein during a high intensity PRT will increase muscle protein synthesisand increase the rate of hypertrophy. Older men and women will consume a eucaloric diet providing the protein RDA of 0.8 or 1.6g for three months. Factoral nitrogen balance along with constant, primed infusion of 1-13C-leucine will allow us to examine whole body amino acid kinetics and incorporation of the label into skeletal muscle to determine the effects of the diet, age, and PRT on the rate of muscle protein synthesis. We have shown that computerized tomography and biopsy analysis are very sensitive tools to examine the rate of muscle hypertrophy. These studies will be the first in which the effects of dietary protein on muscle metabolism, size and strength will be examined in a tightly controlled metabolic ward. This study provides an opportunity to examine the adequacy of the protein RDA in the elderly and the influence of training of N retention. These studies will allow us to make specific nutritional recommendations for the elderly as they initiate and continue with a strength training program.